The emphasis of exploring and understanding the physical properties of spark-processed silicon (sp-Si) has been directed in the past mainly towards its strong, room temperature, photoluminescence (PL) in the blue and green spectral range (R. E. Hummel, in Silicon-Based Materials and Devices, Vol. 1, Materials Processing, edited by H. S. Nalwa (Academic Press, New York, 2001) pp. 237-266, and R. E. Hummel and S.-S. Chang (1992) Appl. Phys. Lett. 61:1965). The usefulness of sp-Si is widely recognized because of the stability of this material towards high-temperature annealing (at least up to 1000° C.), environmental interactions, laser radiation, and HF etching (R. E. Hummel, in Silicon-Based Materials and Devices, Vol. 1, Materials Processing, edited by H. S. Nalwa (Academic Press, New York, 2001) pp. 237-266 and R. E. Hummel and S.-S. Chang (1992) Appl. Phys. Lett. 61:1965). Further, the PL of sp-Si is fast, having decay times in the nanosecond range.
The electroluminescence (EL) properties of sp-Si have also been explored, with however, limited success (J. Yuan and D. Haneman (1995) Appli. Phys. Lett. 67:3328). Specifically, the EL light emission of sp-Si was found to be considerably smaller than that observed for the PL mode.
Conventional spark-processing is performed by applying high frequency, high voltage, low average current electrical pulses for a certain length of time between a substrate and a counter electrode. As an example, pulses can be applied for several seconds between 2 Ω cm, 400 μm thick <100> Si wafer and a counter electrode. The sparks can be applied through the native SiO2 layer while the non-sparked areas remain covered by SiO2. A tungsten tip (anode) has been found to be an efficient counter electrode and can be placed about 0.5 mm above the substrate (cathode) (M. E. Stora and R. E. Hummel (2002) J. Phys. Chem. Sol. 63:1658). Unipolar pulses involving, for example, a frequency of 16 kHz, currents between 5 to 10 mA and air as a sparking medium are typical (M. E. Stora and R. E. Hummel (2002) J. Phys. Chem. Sol. 63:1658). The typical resulting product is a grayish looking layer on (and in) the Si substrate which; in plan view, is surrounded by a light brown halo.
A complete EL device can have a sp-Si layer on a Si substrate, an ohmic aluminum contact on the back side of the wafer, and a thin (15-17 nm thick) semitransparent silver (Ag) film which covers the front (spark-processed) surface, as shown in FIG. 1. The transparency of a smooth Ag film of the aforementioned thickness for 700 nm light is about 30%. However, the actual film thickness over the spark-processed area can vary considerably due to its rough and pitted nature so that different transmissivities should be expected across the spark-processed surface. Moreover, 80% of the sp-surface is probably not continuously covered by the conductive film so that approximately only 20% of the sp surface participates in the EL emission. This is illustrated in FIG. 2 which depicts the EL emission of conventionally spark-processed Si under 30-fold magnification when a driving voltage of 7V is applied to the device. Specifically, to the naked eye, the EL emission can appear to be a continuous circular band of yellowish-red light which emanates only from the halo region. Moreover, under an optical microscope it is observed that the band consists of small, individual, light-emitting spots, which are separated from each other (on the order of tens of microns) by non-emitting areas. Some of these spots emit orange, others green, and still others, blue light. They appear randomly distributed over the emitting surface.
The subject invention relates to a method for spark processing which increases the EL emission of sp-Si by at least one order of magnitude compared to the intensities which are achieved when conventional spark-processing techniques are applied.